1. Field of the Invention
The present invention relates in general to gas discharge lamp lighting systems and, in particular, devices for starting such lamps.
2. Description of the Prior Art
Conventional starting devices for gas discharge lamps such as fluorescent lamps typically include mechanical starting devices which include bimetallic switches operative to control the flow of current through filaments or cathodes of the lamps. As is well known in the art, current flow through such starting devices causes them to open a circuit which includes a transformer ballast. When the circuit is open, a voltage is induced across the ballast, and this ballast voltage is added to the a.c. line voltage in order to provide a starting voltage across the electrodes of the lamp.
However, such conventional systems have certain drawbacks. First of all, because the starter is thermally actuated, there is no means for synchronizing the ballast voltage with the line voltage. The maximum current through the ballast occurs when the a.c. voltage is zero. As a result, the a.c. voltage and the ballast voltage are ninety degrees out of phase when they are added. Further, because there is no means for synchronizing the ballast voltage with the line voltage, such starters will only randomly maximize the starting voltage. Thus, the lamp may require several starter operations before ignition.
Another problem with conventional gas discharge or fluorescent lamps is that the reduced temperatures found in outdoor applications cause a corresponding reduction in gas pressure inside the lamp. This requires higher striking or igniting voltages to ignite or start the lamp.
Still another problem with conventional lighting fixtures is the control of preheat current. In particular, many preheat type lamps do not properly turn off the preheat current after ignition of the lamp. This can cause damage to the filaments and decrease the useful life of the lamp.
Some devices known in the prior art have attempted to overcome these problems. For example, high frequency electronic ballast circuits have been provided to start fluorescent lamps in cold ambient temperatures. However, such circuits typically generate high radio frequency signals which can interfere with radio frequency transmission and reception. Attempts have been made to eliminate such radio frequency interference by grounding and shielding such circuits, but this approach requires additional expense and is sometimes unsatisfactory.